Well (as others have stated) the in game notes say that starfleet gave captains a bit of leeway to pick uniforms they prefer. While this was obviously so people would actually play the game, I like to think it shows that starfleet have taken a less formal approach and now allow everyone to dress up as firefly characters or whatever...
If I were to pick the one I thought was most appropriate for the official uniform, I'd have to go for Enterprise. It's the only one that looks like you could actually work in it.
The day will not save them. And we own the night.
For what it's worth, the in-game Sierra uniform are the same as the Starfleet uniforms seen in the Countdown comic, which was the prequel to the 2009 movie, telling the story of Spock and Nero's efforts to stop the Hobus supernova. A lot of STO's continuity is based on the events of that comic series.
I prefer the Sierra 1 uniform out of the choices for 25th Century uniforms available w/o C-Store purchases. In my opinion, the Sierra 1 design maintains the style of previous uniforms all the way back to TNG while showing a progression in design consistant with changing times.
Doesn't mean I'm not above using other uniform styles such as TNG, WoK, or even Enterprise. However... for my BOffs... its Sierra 1 all the way.
TNG-movie/later DS9 uniforms now and forever, followed closely by WoK. If only I could see all NPCs dressed as either
I know that feel bro.
That being said though, the Odyssey uniform is very well done, and actually looks like something that would actually appear in a Star Trek series. I would be very happy if the Odyssey were the Default for all the NPC's. The Antares and Sierra sets deserve to burn.